The present invention relates to a measuring method and a measuring instrument which measures a multiple number of points periodically or continuously over time
For the purposes of observing a sinking of ground, of observing a landslide, of observing a displacement of banks and walls of construction work such as a dam, of observing a displacement of wall surface of a tunnel, a multiple number of measuring points set in a measurement range are measured periodically or continuously over time by a measuring instrument.
For instance, in a case where a tunnel is dug for a construction of an underground railway, for the purpose of observing whether or not the ceiling and the wall surface of the tunnel constructed may not be displaced, a multiple number of prisms are installed as targets on the ceiling and on the wall surface and positional displacement of all the prisms are measured one after another without operator intervention by the measuring instrument (hereinafter referred as “monitoring measurement”).
When the monitoring measurement is performed, the measuring instrument automatically searches the prisms, i.e. the measuring points, performs sighting, measures distances and carries out the measuring operation. In order to acquire initial values to search the measuring points as set up, it is necessary to carry out teaching operations on each of the measuring points.
In a conventional teaching operation, an operator sights each of the measuring points by a measuring instrument, measures a horizontal angle, vertical angle and distance for each of the measuring points, performs three-dimensional measurement with respect to each of the measuring points and sets the results of measurement thus obtained as initial values of each measuring point respectively. According to this method, the operator measures the measuring points one by one, and further, a view angle of a sighting telescope provided on the measuring instrument is narrow. This means that much time is required for sighting and the working efficiency is low. As a result, much time is required. In particular, in a case where there are restrictions in terms of time for the measuring operation, e.g. in a case where the operation is performed in a tunnel for an underground railway, the measuring operation must be completed during the time period when the underground railway is not in operation in order to perform monitoring measurement on the ceiling and the wall surface of the tunnel, and so the measuring time is required to be shortened.
In the Japanese Patent Application Publication. JP-A-2012-73201, the present applicant discloses a measuring method and a measuring instrument which extensively shortened the teaching operation time by acquiring images at a predetermined time interval while scanning over a preset range, detecting prisms from the images acquired, acquiring an initial value to perform searching by obtaining a coordinate position of the prism (measuring point) and carrying out teaching with respect to each of the prisms.
In the measuring method and the measuring instrument according to the Japanese Patent Application Publication JP-A-2012-73201, prisms are detected from the images and the coordinate position of the image center of the prism is measured. Because of this, if an imperfect condition, where a part of image is missing or the like, there is a case where the coordinate position of the image center may have an error with respect to the coordinate position of the prism.